A polishing method for removing spherical crown-shaped protrusions on the surface of optical film nodule defects

Abstract

The present invention relates to the field of optical thin films, and is mainly aimed at nodule defects that reduce the damage threshold of optical thin films, and specifically relates to a method for increasing the laser damage threshold of thin films based on removing spherical crown-shaped protrusions on the surface of nodule defects. The method provided by the present invention comprises: using a smooth surface with a roughness less than that of a thin film element as a polishing surface; using a liquid with good wettability to form a liquid film between the polishing surface and the film surface, and the capillary force generated by the liquid film is used as a positive pressure acting on the film surface; the nodule protrusions on the film surface are removed by friction during the relative sliding process between the polishing surface and the film surface. The present invention uses a smooth surface and a liquid with good wettability to remove the nodule protrusions on the film surface, thereby reducing the electric field enhancement caused by nodule defects, and improving the laser damage threshold of thin film elements without affecting the spectral performance of the film. Compared with the existing methods for increasing the laser damage threshold of optical thin films, the present invention has the characteristics of simple operation, low cost and wide application range.

Description

Technical Field

[0001] The invention relates to an optical film, in particular to a polishing method for removing spherical crown-shaped protrusions on the surface of nodule defects of an optical film. Background Art

[0002] With the continuous development of laser technology, optical films have been increasingly widely and importantly used in laser systems. However, optical films are the weakest link in laser systems that are most susceptible to damage and are the key factor limiting the increase in laser system power. High-power laser systems such as the Shenguang series laser devices in China and the National Ignition Facility (NIF) in the United States continue to place higher requirements on the laser damage resistance of optical films. A large number of theoretical and experimental studies conducted by researchers at home and abroad have shown that nodule defects are one of the main factors that reduce the laser damage threshold of thin film components such as high-reflection films in nanosecond laser systems (Light Sci. Appl. 2013, 2, e80). Nodule defects are inverted cone defects with spherical crown-shaped protrusions formed by particle growth. The spherical crown protruding from the surface of the film will change the incident angle of the local incident light, thereby causing a change in the transmittance, and then causing the electric field distribution of the nodule defect and the film layer nearby to change, resulting in an increase in the local electric field intensity. Higher electric field intensity in the local position of the laser irradiation area will more easily induce film damage, thereby reducing the laser damage threshold of the thin film component. At present, in order to reduce the damage resistance of thin films due to nodule defects, the following two methods are mainly used to improve the laser damage threshold of thin films:

[0003] One is to reduce the generation of nodule defects or suppress the eruption of nodule defects by optimizing the coating process. For example, using metal hafnium instead of hafnium oxide as the initial coating material can reduce the density of nodule defects in the film layer, but this method can only reduce the seed source of defects to a certain extent, and the nodule defects still limit the film threshold. Using ion beam thin film smoothing technology, the convex defects of the substrate can be effectively smoothed through multiple cycles of deposition-etching to suppress the generation of nodule defects, but this method is complex and time-consuming, and it is not easy to achieve uniform etching of large-diameter components. In addition, the eruption of nodule defects can be suppressed by optimizing the electric field distribution at the nodule defect through film system design and improving the continuity of the nodule defect boundary through ion beam assisted deposition, but nodule defects are still the key to limiting the further improvement of the damage threshold of thin film components.

[0004] Second, the use of nanosecond laser pretreatment technology can reduce the probability of nodule defects inducing film damage. On the one hand, nodule defects with poor boundary bonding will spontaneously erupt during laser pretreatment, leaving relatively stable nodule pits, reducing the possibility of nodule defects inducing serious film damage during use; on the other hand, the thermal effect of nanosecond laser irradiation may improve the boundary conditions of nodule defects with poor continuity to a certain extent, and nodule defects are thermally reinforced, so their eruption is suppressed. However, laser pretreatment has problems such as incomplete removal of low-threshold defects, different pretreatment processes for different films, large differences in pretreatment effects, and a long pretreatment process. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a method for improving the damage threshold of optical thin films based on removing the spherical crown-shaped protrusions on the surface of nodule defects. The method provided by the present invention includes: using a smooth surface with a roughness smaller than that of a thin film element as a polishing surface; using a liquid with good wettability to form a liquid film between the polishing surface and the film surface, and the capillary force generated by the liquid film acts as a positive pressure acting on the film surface; the nodule protrusions on the film surface are removed by friction during the relative sliding process between the polishing surface and the film surface. The present invention uses a smooth surface and a liquid with good wettability to remove the nodule protrusions on the film surface, thereby reducing the electric field enhancement caused by nodule defects and improving the laser damage threshold of the thin film element without affecting the spectral performance of the film. Compared with the existing methods for improving the laser damage threshold of optical thin films, the present invention has the characteristics of simple operation, low cost and wide application range.

[0006] The technical solution of the present invention is as follows:

[0007] A polishing method for removing spherical crown-shaped protrusions on the surface of optical film nodules, which is characterized by: using a smooth surface with a roughness smaller than that of a film element as a polishing surface; using a liquid with good wettability to form a liquid film between the polishing surface and the film surface, and the capillary force generated by the liquid film acts as a positive pressure acting on the film surface; the nodules on the film surface are removed by friction during the relative sliding process between the polishing surface and the film surface.

[0008] According to the above method for removing spherical crown-shaped protrusions on the surface of nodule defects, the smooth surface is a smooth surface of crystal or glass.

[0009] According to the above method for removing spherical crown-shaped protrusions on the surface of nodule defects, the contact angles between the liquid with good wettability and the film surface and the polished surface are less than 90°, and no chemical reaction occurs with the film surface and the polished surface.

[0010] According to the above method for removing the spherical crown-shaped protrusions on the surface of the nodule defect, the capillary force originates from the pressure difference (additional pressure) between the inside and outside of the liquid film, and increases as the thickness of the liquid film becomes thinner.

[0011] According to the above method for removing the spherical crown-shaped protrusions on the surface of the nodule defect, the preparation method comprises the following steps:

[0012] ① Cleaning: Ultrasonic cleaning of the polished surface in deionized water to remove impurity particles adsorbed on the polished surface;

[0013] ② Wetting: Add liquid with good wettability to the polishing surface, and the liquid will spread quickly on the polishing surface;

[0014] ③ Main polishing: Place the film surface on the polishing surface, form a uniform liquid film between the film surface and the polishing surface, push the film element to rotate at a uniform speed on the polishing surface, the liquid film gradually becomes thinner during the rotation, the capillary force between the film surface and the polishing surface gradually increases, and the nodules on the film surface are gradually removed by friction during the rotation;

[0015] ④ Cleaning: Ultrasonic cleaning of the polished surface in deionized water, and washing of the film surface with deionized water to remove most of the abrasive particles generated during the main polishing process;

[0016] ⑤ Fine polishing: Place the film surface on the polishing surface under water flow, push the film element to rotate at a constant speed on the polishing surface, and the abrasive particles adsorbed on the film surface during the main polishing process are gradually removed during the rotation process;

[0017] ⑥ Drying: Place the film element under a high temperature baking lamp to dry it.

[0018] Technical effects of the present invention:

[0019] 1. The present invention utilizes a smooth surface and a liquid with good wettability to remove nodules on the surface of the film, thereby reducing the electric field enhancement caused by nodule defects and significantly improving the laser damage threshold of the thin film element.

[0020] 2. In the process of removing the nodules and protrusions, the present invention does not introduce abrasive particles and no chemical reaction occurs. Only the nodules and protrusions are effectively removed, and the spectral performance of the thin film element will not be affected.

[0021] 3. The method of the present invention is economical, easy to implement, and simple to operate, and is suitable for optical thin film elements of different sizes and prepared by different deposition processes.

[0022] 4. The present invention has a wide range of applications, and is not only applicable to high-reflection films whose thresholds are greatly affected by nodules, but also to other optical film elements whose damage thresholds are affected by nodules. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is an electron microscopic morphology of the nodule defect and the nodule defect with the spherical crown protrusion removed. Figure 1 (a) is the electron microscopic morphology of the nodule defect; Figure 1(b) is an electron microscopic morphology of the nodule defect after the spherical crown-shaped protrusions were removed.

[0024] Figure 2 The laser damage probability curves are of a thin film sample with nodule defect spherical crown protrusions removed by the method of the present invention and an untreated sample, and the laser wavelength is 355nm.

[0025] Figure 3 It is a cross-sectional view of a nodule defect and a cross-sectional view of a nodule defect with the spherical crown-shaped protrusion removed. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with the embodiments and drawings.

[0027] Example

[0028] Taking the incident angle of 45 degrees, the reflectivity of the s-polarized component at 355nm>99.5%, and the UV reflective film (roughness of about 2nm) with a pre-implanted nodule defect seed source of 550nm silica microspheres as an example, the method of improving the laser damage threshold of the film based on removing the spherical crown protrusions on the surface of the nodule defect of the present invention is explained. The smooth surface is selected from the polished surface of a silicon wafer with a diameter of 8 inches (roughness less than 0.5nm), and the liquid with good wettability is selected from anhydrous ethanol. Figure 1 The electron microscopic morphology of the nodule defect and the nodule defect with the spherical crown protrusion removed. The method comprises the following steps:

[0029] ① Cleaning: Ultrasonic cleaning of silicon wafers in deionized water for 5 minutes to remove impurity particles adsorbed on the silicon wafers;

[0030] ② Wetting: Add anhydrous ethanol to the polished surface of the silicon wafer, and the ethanol will spread rapidly on the polished surface;

[0031] ③ Main polishing: Place the film surface on the polished surface of the silicon wafer, and form a uniform ethanol liquid film between the film surface and the polished surface of the silicon wafer, and push the film element to rotate at a uniform speed on the polished surface of the silicon wafer. During the rotation, the ethanol liquid film gradually becomes thinner, and the capillary force between the film surface and the polished surface of the silicon wafer gradually increases. The nodules on the film surface are gradually removed by friction during the rotation;

[0032] ④ Cleaning: Ultrasonic cleaning of silicon wafer in deionized water for 5 minutes, and washing of film surface with deionized water for 5 minutes to remove most of the abrasive particles generated during the main polishing process;

[0033] ⑤ Fine polishing: Place the film surface on the polishing surface of the silicon wafer under water flow, and push the film element to rotate at a constant speed on the polishing surface of the silicon wafer. The abrasive particles adsorbed on the film surface during the main polishing process are gradually removed during the rotation process;

[0034] ⑥ Drying: Place the thin film element under a high temperature baking lamp and bake for about 5 minutes.

[0035] ⑦Spectral performance measurement:

[0036] Lambda 1050 spectrophotometer was used to measure the spectrum of the optical film treated and untreated by the method of the present invention. The incident angle was 45°, s polarization component, measurement wavelength: 300nm-1200nm, test quantity: transmittance value. The reflectance value was obtained by subtracting the transmittance value from 100%.

[0037] The test results show that the spectral performance of the thin film element remains unchanged after being processed by the method of the present invention.

[0038] ⑧Laser damage threshold measurement:

[0039] According to the ISO21254 test standard, the optical film treated and untreated by the method of the present invention is tested by a 1-on-1 test method. Pulse width: 8ns, incident angle: 45°, spot area: 0.30mm 2 , polarization state: s component.

[0040] The test results show that the damage threshold of the optical film treated by the method of the present invention is significantly improved. Figure 2 The laser damage probability curves are of a thin film sample with nodule defect spherical crown protrusions removed by the method of the present invention and an untreated sample, and the laser wavelength is 355nm.

Claims

1. A polishing method for removing spherical crown-shaped protrusions on the surface of optical film nodules, characterized in that: A smooth surface with a roughness smaller than that of the thin film element is used as the polishing surface; a liquid with good wettability is used to form a liquid film between the polishing surface and the thin film surface, and the capillary force generated by the liquid film acts as a positive pressure acting on the thin film surface; The nodules on the film surface are removed by friction during the relative sliding process between the polished surface and the film surface; the contact angles between the liquid with good wettability and the film surface and the polished surface are less than 90°, and no chemical reaction occurs with the film surface and the polished surface.

2. The polishing method for removing spherical crown-shaped protrusions on the surface of optical film nodule defects according to claim 1, characterized in that: The polished surface is the smooth surface of crystal or glass.

3. The polishing method for removing spherical crown-shaped protrusions on the surface of optical film nodule defects according to claim 1 or 2, characterized in that: The method comprises the following steps: ① Cleaning: Ultrasonic cleaning of the polished surface in deionized water to remove impurity particles adsorbed on the polished surface; ② Wetting: Add liquid with good wettability to the polishing surface, and the liquid will spread quickly on the polishing surface; ③ Main polishing: Place the film surface on the polishing surface, form a uniform liquid film between the film surface and the polishing surface, push the film element to rotate at a uniform speed on the polishing surface, the liquid film gradually becomes thinner during the rotation, the capillary force between the film surface and the polishing surface gradually increases, and the nodules on the film surface are gradually removed by friction during the rotation; ④ Cleaning: Ultrasonic cleaning of the polished surface in deionized water, rinse the film surface with deionized water to remove most of the abrasive particles generated during the main polishing process; ⑤ Fine polishing: Place the film surface on the polishing surface under water flow, push the film element to rotate at a constant speed on the polishing surface, and the abrasive particles adsorbed on the film surface during the main polishing process are gradually removed during the rotation process; ⑥ Drying: Place the film element under a high temperature baking lamp to dry it.

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